Various techniques have heretofore been used to measure the thickness of a thin, light transmitting film on a reflecting substrate. Usually they involve measuring the intensity of the polarized light transmitted through or reflected from the film at an angle. These ellipsometric techniques and other methods based upon optical interferometry and also X-ray fluorescence techniques tend to be unduly time consuming. X-ray fluorescence techniques are also incapable of measuring very thin inhomogeneous films or coatings having thicknesses as low as 50 nanometers. Moreover, most are incapable of mapping variations in the thickness of the film over its entire surface due to limitations in speed and/or lateral resolution.
Porosity of a thin film or coating has heretofore been obtained using density measurements and gas adsorption. These techniques provide only an overall picture of porosity. They are not appropriate for measuring local variations in porosity over the entire area of the film or coating and are too time consuming for on-line inspection.
The most pertinent prior art of which applicants are aware is U.S. Pat. No. 4,129,781, which discloses an arrangement for measuring the thickness of a thin film using collimated light impinging at Brewster's angle. However, this patented arrangement will not provide satisfactory measurements on inhomogeneous thin films or coatings which isotropically scatter light. It claims to satisfactorily measure thickness of a film or coating on a striated substrate using absorption measurements; but even using such grooved substrates, the accuracy of the measurement is limited because there will always be some light scattering, and light scattering is not taken into account.
U.S. Pat. No. 3,985,447 discloses another method for measuring the thickness of a thin film using polarized light which impinges on the film at the Brewster's angle of the film. This and other ellipsometric methods examine the phase differences between s and p polarizations and rely on interference effect; however, these techniques can provide spurious results in the presence of diffused or scattered light.
The IBM Technical Disclosure Bulletin dated January, 1978, at p. 3133, discloses an instrument capable of measuring individual layer thicknesses of a composite film by measuring the relative intensity of the s and p components of polarized light reflected at Brewster's angle, as the wavelength of the incident light is varied.
None of these arrangements involving impinging light at the Brewster's angle of the film employ an integrating sphere; and all are capable only of measuring the thickness of the film.
U.S. Pat. No. 4,626,101 discloses a laser beam directed either vertically or at a slight angle off vertical into an integrating sphere that has an opening in its lower portion to permit light to be reflected back into the sphere from a specimen to be examined. The intensity of the scattered light collected on the reflective surface within the sphere is detected, converted from an analog to a digital value, and compared with a reference value to classify the digital data on the basis of the size of a surface defect. An R-.theta. turntable is provided to enable sampling the entire surface of the specimen. However, this arrangement does not disclose or suggest having the laser beam impinge at the Brewster's angle of the specimen to minimize interference effects and thus can not measure coating thickness.
There is a need for an improved method and apparatus which permits on-line inspection of a thin film by optically measuring various characteristics, such as the thickness, porosity and surface roughness of the film.